Electronic Design - What knowledge is required for PCB design?

What knowledge is required for PCB design?

PCB design engineers need to master a wide range of knowledge systems, covering electrical theory, component performance, digital circuits and analog circuits, PCBA processing technology and DFM manufacturability theory, welding practice operations, schematic drawing and layout, microcontroller program logic and Basic principles, etc.

1. If the designed circuit system contains FPGA devices, the Quartus II software must be used to verify the pin assignments before drawing the schematic. (Some special pins in FPGA cannot be used as ordinary IO).

2. The 4-layer board from top to bottom is: signal plane layer, ground, power, signal plane layer; the 6-layer board from top to bottom is: signal plane layer, ground, signal inner electric layer, signal inner electric layer, Power and signal plane layer. For boards with 6 layers or more (the advantage is: anti-interference radiation), the internal electrical layer wiring is preferred, and the plane layer is not allowed to go. It is forbidden to wire from the ground or power layer (reason: the power layer will be divided, resulting in parasitic effects).

3. Wiring of multi-power supply system: If FPGA+DSP system is made of 6-layer board, there will be at least 3.3V+1.2V+1.8V+5V.

3.3V is generally the main power supply, and the power layer is directly laid, and it is easy to route the global power network through vias;

5V may generally be the power input, and only a small area of copper is required. And as thick as possible.

1.2V and 1.8V are the core power supply (if you directly use the wire connection method, you will encounter great difficulties when facing BGA devices). Try to separate 1.2V and 1.8V during layout, and let 1.2V or 1.8V connect The components are arranged in a compact area and connected by copper

In short, because the power supply network is spread over the entire PCB, it will be very complicated and long to go around if it is routed. The method of laying copper is a good choice!

4. The wiring between adjacent layers adopts a cross method: it can reduce electromagnetic interference between parallel wires and facilitate wiring.

5. What is the isolation method for analog and digital isolation? Separate the devices used for analog signals from those used for digital signals during layout, and then cut across the AD chip across the board!

The analog signal is laid with an analog ground, and the analog ground/analog power supply and the digital power supply are connected at a single point through an inductor/magnetic bead.

6. PCB design based on PCB design software can also be regarded as a software development process. Software engineering pays most attention to the idea of "iterative development" to reduce the probability of PCB errors.

(1) Check the schematic diagram, pay special attention to the power and ground of the device (power and ground are the blood of the system, and there can be no negligence);

(2) PCB package drawing (confirm whether the pins in the schematic diagram are wrong);

(3) After confirming the PCB package size one by one, add a verification label and add it to the package library of this design;

(4) Import the netlist, adjust the signal sequence in the schematic while laying out (the OrCAD component automatic numbering function can no longer be used after the layout).

In the specific design process, the basic knowledge that needs to be mastered includes:

1. Preliminary preparation

Including the preparation of component libraries and schematic diagrams. Before proceeding with PCB design, we must first prepare the schematic SCH component library and PCB component packaging library.

The PCB component package library is best established by the engineer based on the standard size data of the selected device. In principle, establish the PC component package library first, and then establish the schematic SCH component library.

PCB component package library requirements are high, which directly affects the installation of PCB; schematic diagram SCH component library requirements are relatively loose, but pay attention to define the pin attributes and the corresponding relationship with the PCB component package library.

2. PCB structure design

According to the determined circuit board size and various mechanical positioning, draw the PCB frame in the PCB design environment, and place the required connectors, buttons/switches, screw holes, assembly holes, etc. according to the positioning requirements.

Fully consider and determine the wiring area and non-wiring area (such as how much area around the screw hole belongs to the non-wiring area).

3. PCB layout design

The layout design is to place the components in the PCB frame according to the design requirements. Generate the netlist (Design-CreateNetlist) in the schematic tool, and then import the netlist (Design-ImportNetlist) in the PCB software. After the netlist is successfully imported, it will exist in the background of the software. Through the Placement operation, all devices can be called out, and there is a flying line prompt connection between the pins. At this time, the layout design of the device can be carried out.

PCB layout design is the first important process in the entire PCB design process. The more complex the PCB board, the better the layout can directly affect the difficulty of later wiring.

Layout design depends on the circuit board designer's basic circuit knowledge and rich design experience, which is a higher level requirement for the circuit board designer. Elementary circuit board designers have little experience and are suitable for small module layout design or PCB layout design tasks with lower overall board difficulty.

4. PCB wiring design

PCB layout design is the process with the largest workload in the entire PCB design, which directly affects the performance of the PCB board.

In the PCB design process, wiring generally has three realms:

The first is the distribution, which is the most basic entry requirement for PCB design;

The second is the satisfaction of electrical performance, which is a standard to measure whether a PCB board is qualified. After the wiring is laid out, carefully adjust the wiring to achieve the best electrical performance;

Thirdly, neat and beautiful, chaotic wiring, even if the electrical performance is passed, it will cause great inconvenience to the later modification of the board optimization and testing and maintenance. The wiring requirements are neat and uniform, and they cannot be crisscrossed and disordered.

5. Wiring optimization and silk screen placement

"PCB design is not the best, only better", "PCB design is an art of defects", this is mainly because PCB design needs to realize the design requirements of all aspects of hardware, and individual requirements may be in conflict with each other. Bear's paw can't have both.

For example: a PCB design project needs to be designed as a 6-layer board after evaluation by the circuit board designer, but the product hardware must be designed as a 4-layer board due to cost considerations, so the signal shielding ground layer can only be sacrificed, resulting in adjacent wiring The signal crosstalk between layers increases and the signal quality decreases.

The general design experience is: the time to optimize the wiring is twice the time of the first wiring. After the PCB layout optimization is completed, post-processing is required. The first thing to do is the silk screen logo on the PCB surface. The bottom silk screen characters need to be mirrored during design to avoid confusion with the top silk screen.

6. Network DRC inspection and structure inspection

Quality control is an important part of the PCB design process. General quality control methods include: design self-inspection, design mutual inspection, expert review meetings, special inspections, etc.

Schematic diagram and structural element diagram are the most basic design requirements. Network DRC inspection and structural inspection are to confirm that the PCB design meets the two input conditions of the schematic netlist and structural element diagram.

Generally, circuit board designers will have their own accumulated design quality check Checklist, where the entries partly come from the company or department's specifications, and the other part comes from their own experience summaries. Special inspections include Valor inspection and DFM inspection of the design. These two parts focus on the PCB design and output back-end processing gerber files.

7. PCB system board

Before the PCB is formally processed and manufactured, the circuit board designer needs to communicate with the PE of the PCB supplier to answer the manufacturer's confirmation questions about the PCB board processing.

This includes but is not limited to: selection of PCB boardmodel, adjustment of circuit layer line width and line spacing, adjustment of impedance control, adjustment of PCB stacking thickness, surface treatment processing technology, aperture tolerance control and delivery standards, etc.